ABRASIVE BLASTING, SURFACE PROFILE

Not only inorganic zinc coatings and solventfree coatings, but most paint systemsrequire a roughened substrate surface to obtain proper adhesion. The surfaceprofile of the roughened substrates is characterized by a surface roughness and aroughness profile, which must be itemized separately in specifications for surfacepreparation.

During field work the anchor pattern is conveniently assessed by visual or tactilecomparison, using standardized comparators. Such comparators are e.g. RugotestNo. 3, Keane-Tator Surface Profile Comparator, and ISO 8503 surface profilereference comparators.

Surface roughness: In connection with surface preparation, roughness is defined as the irregularities insurface texture, which are caused by blastcleaning.

The roughness can be characterized by several roughness values. Most often theroughness is designated by the maximum height of the profile (peak-to-valleyheight), RZ . Sometimes the arithmetical mean deviation of the profile RA,previously known as CLA- and AA-values (Centre Line Average and ArithmeticalAverage, respectively), is used. Designations in boldface are according to ISOStandard.

Because these values may have very different numerical values assigned to them fora given surface, it is very important to distinguish between them.

It is also important to note that roughness comparison specimens may use differentroughness values. Rugotest No. 3 uses roughness numbers according to ISO 1302and 2632/II, which are assigned to RA values. Keane-Tator Surface ProfileComparator uses the maximum average peak-to-valley height, which ressembles RZ,while ISO surface profile reference comparators uses the designations Fine,Medium, and Coarse.

Although it is not possible to calculate RA values from RZ and vice versa, a workinggroup of the international Standards Sub-Committee TC 35/SC 12 has establishedthat a good approximation for RZ is RA x 6.

Roughness profile: Roughness profiles can be characterized as round or sharp edged. Steel shotproduces a round profile, while not worn down steel grit as well as most mineralabrasives give a sharp edge.

When a roughness profile is given in Hempels Product Data Sheets it is normally asharp profile.

Because optical effects play a role when judging a surface by means ofcomparators, both Rugotest No. 3, Keane-Tator Surface Profile Comparator, and ISOsurface profile reference comparators have different scales for different profiles.

Rugotest No. 3 has specimens for round and sharp profiles collected in onecomparator. For greater roughness values there is even a division in fine andcoarse grained finish.

Keane-Tator Surface Profile Comparator has three different discs, designed by S(sand), G/S (steel or metallic grit), and SH (shot), respectively. ISO Comparators areobtainable either as a G version or a S version for use on gritblasted andshotblasted surfaces, respectively.

The disc corresponding to the abrasive used must be selected for comparison.

HEMPEL Product Data